The present invention relates to combined cycle electric power plants and more particularly to improved gas turbine overspeed protection controls especially useful in the operation of gas turbines in combined cycle electric power plants.
In the operation of gas turbines in electric power plants, it is necessary to protect the turbine structure reliably from damage that could result from overspeed. Typically, if the speed of a power plant gas turbine reaches a predetermined overspeed value such as 111% of rated speed, an overspeed trip mechanism causes fuel shutoff to shut down the turbine with resultant loss of generation capacity. Such a trip mechanism can include for example apparatus which detects overspeed by some mechanical means and then causes the closure of an overspeed trip valve in the fuel line which supplies fuel to the nozzles in the turbine combustors. It could further include control circuitry which generates an electric signal to cause the valve position control to close the isolation valve and trip the turbine if the turbine speed exceeds some speed slightly below the mechanical overspeed setpoint.
When the turbine is shut down, it cannot normally be restarted until after it has coasted to a complete stop. The shut down and restarting process typically takes about 30 minutes or more and it is therefore undesirable to allow the turbine to reach the trip level unless there is a serious malfunction which renders a shut down mandatory to prevent serious damage to the turbine. It has therefore also been customary to provide electrical overspeed protection through the throttle valve fuel control so as to hold the turbine at some lower overspeed value such as 104% rated speed during power system overspeed conditions or during isolated operation of the gas turbine.
During gas turbine startup, fuel flow is typically controlled by a startup control which either directly controls speed or indirectly controls speed by controlling fuel flow to satisfy an acceleration control, an exhaust temperature limit control, and/or a surge limit control. During electrical load operation, fuel flow can be controlled to generate a particular load under load control within exhaust temperature limits, or to gain the advantage of higher heat rates the fuel can be controlled by a temperature limit control to produce turbine operation at the maximum load level permitted by the blade path or exhaust temperature limit. The speed and load controls are generally channeled together where both are provided to form a speed/load control, and temperature limit and other constraint controls are channeled into the speed/load control so that the lowest control signal operates as a fuel demand on an electropneumatic control which positions the fuel throttle valve. The control may be the relay-pneumatic type, the electronic analog type or a digital/analog hybrid type. Reference is made for example to U.S. Pat. No. 3,520,133 issued to A. Loft on July 14, 1970 where typical electronic control apparatus is described.
To obtain first level electrical overspeed protection, the speed control loop can be employed if it is provided, i.e. once the breaker opens during the load mode, the speed control cuts back on the fuel demand by the operation of the speed control loop through the speed/load control, or during power system overspeed in the load mode the speed/load control similarly cuts back on the fuel demand. Turbine overspeed protection obtained by normal functioning of the speed/load control, such as that associated with the gas turbine computer control system disclosed in the above referenced Ser. No. 319,114, provides advantages over the case of overspeed protection by direct mechanical and/or electrical trip alone since costly shut downs and temporary loss of generation capacity are avoided in situations where generation shut downs are unnecessary, but overspeed protection through the speed/load control does have a response time associated with it due to the operation of included transfer functions in the speed control loop with or without a speed reference change for the speed control loop upon a switch from the load control mode to the speed control mode. That response time causes an undesirable bump in turbine response just after the breaker opens, i.e. when the load is dumped the turbine speeds up as the fuel flow is controllably reduced by the reaction of the speed/load control.
In prior steam turbine overspeed protection systems, electrical overspeed protection systems may have been employed independently of speed/load control and independently of the mechanical or electrical overspeed trips to achieve an electrical cutback in turbine energization under a first range of overspeed conditions and to achieve a mechanical or electrical trip of the steam turbine at upper limiting overspeed conditions. However, the prior steam turbine control art provides no disclosure on how the principles of independent electrical overspeed protection can be beneficially applied to combinations of gas turbine control system elements.
In combined cycle electric power plants, gas turbine overspeed protection is especially important if the plant is designed to function without system frequency participation. In that event, unnecessary gas turbine trips are desirably avoided on the electrical isolation of the combined cycle plant since substantial generation capacity represented by the rest of the plant can otherwise be lost by plant shut down. Thus, there is a need for improved gas turbine electrical overspeed protection, especially where gas turbines operate with other apparatus in combined cycle plants.
The description of prior art herein is made on good faith and no representation is made that any prior art considered is the best pertaining prior art nor that the interpretation placed on it is unrebuttable.